This invention relates to television cathode ray picture tubes and the electron guns used therein, and is particularly concerned with improving the performance of such guns.
An in-line electron gun generates three coplanar electron beams consisting of a center beam and two adjacent off-axis beams. The center beam is normally utilized to excite the green phosphor targets on the viewing screen of the color cathode ray tube, and the two off-axis beams are normally used to excite the red and blue phosphor targets. The three beams are caused to scan the viewing screen in unison, projecting a substantially rectangular raster pattern having an aspect ratio of, normally 3:4. The beams are caused to scan by the influence of the fields produced by a deflection yoke which encircles the neck of the tube. The center beam, which lies on the tube axis, is subject to symmetrical yoke fields, while the two adjacent beams are subject to asymmetrical fields because of their off-axis location. As a result, the raster pattern projected by the center beam may differ in size from the superimposed raster patterns projected by the off-axis beams. The resulting aberration is known as "coma distortion."
Typical means to correct coma distortion are disclosed in U.S. Pat. No. 4,196,370 to Hughes and in UK Patent Application No. GB 2 020 480A to RCA. In one embodiment of the '370 patent, thin, washer-like magnetic shield members completely surround the outer beams. In another embodiment, shunts surrounding the outer beams are offset eccentrically toward the center beam relative to the outer beams. In the '480 A application disclosure, the raster correction means comprises, in one embodiment, thin, washer-like rings surrounding the outer beams, and two "rail-shaped" elements perpendicularly oriented to the plane of the three beams, and located between the beams.
A problem arises in the use of such washer-like elements in that the range of beneficial effects is limited. For example, increasing the thickness of the material has a diminishing effect. If the elements are made smaller to weaken the effect, the elements become so small as to be difficult to handle and install during manufacture.
Takenaka et al in U.S. Pat. No. 3,860,850 is directed to a set of shunts and enhancers located in the cathode ray tube between the output end of the electron gun and the deflection yoke. A pair of horseshoe-shaped shunting elements having high magnetic permeability surround the outer beams of the three-beam in-line gun and have their open ends vis-a-vis. The effect of the shunting is to shield the outer beam from a portion of the horizontal magnetic field such that the outer beam rasters are contracted relative to the raster traced by the center beam. In addition, two enhancers each having a V-shape are disposed above and below the plane of the electron beam; these enhancers act to concentrate the horizontal deflection field in the region of the center beam so as to expand the horizontal dimension of the raster formed with the center beam. The V-shaped enhancers also act to dilute the vertical deflection field in the region of the center beam so as to contract the vertical dimension of the center beam raster relative to the outer beam rasters.
Murata et al in U.S. Pat. Re. No. 28,895 discloses a cathode ray tube with an in-line gun wherein an electron beam control device in the form of a magnetically soft structure partially surrounds the outer beams to reduce the size of the rasters produced by the outer beams so as to bring them into coincidence with the center beam raster. Two devices in the form of horse shoes vis-a-vis surround the outer beams in a manner very similar to that shown by Takenaka et al. Also as with Takenaka et al, the two devices act as shunts to shield the outer beams from a portion of the horizontal magnetic field, and hence contract the rasters projected by its outer two beams.
In U.S. Pat. No. 3,548,249 to Yoshida et al, there are disclosed horse-shoe-shaped elements located adjacent the center beam but facing outwardly so as to partially enclose the outer beams. As with the similarily shaped elements of Takenaka et al and Murata et al, the elements act as shunts to reduce the size of the rasters of the outer beams into coincidence with the center beam rasters.